1. Field of the Invention
The present invention relates to a method of manufacturing a crystalline silicon film and a method of manufacturing a semiconductor device using the crystalline silicon film.
2. Description of the Related Art
A thin film transistor (hereinafter referred to as a TFT) using a silicon film formed on an insulating surface has been known.
As a structure of a thin film transistor presently put into practical use, there are a one using an amorphous silicon film and a one using a crystalline silicon film named high temperature polysilicon.
Since the one using the amorphous silicon film can use a glass substrate, it has been extensively used for an active matrix type liquid crystal display device.
However, since a TFT using an amorphous silicon film has low electrical characteristics, it does not have especially a field of application except being used for an active matrix circuit.
On the other hand, for the purpose of incorporating a driving circuit in a liquid crystal panel or obtaining a higher display function, it is required to obtain a TFT having higher characteristics. In addition, for the purpose of widening the range of application in addition to the active matrix, a TFT having characteristics higher than a TFT using an amorphous silicon film is required.
A high temperature p-Si is obtained by using a technique for obtaining a crystalline silicon film, which uses a heat treatment at a high temperature such as 900° C. or more.
In view of the requirements for high characteristics, it is preferable to use a crystalline silicon film. However, there is a problem that a glass substrate can not be used as a substrate at a temperature of heat treatment required for manufacturing the high temperature p-Si.
A thin film transistor is mainly used for an LCD device, and it is required that a glass substrate can be used as a substrate.
In addition, for the purpose of reducing a process margin and manufacturing cost, it is required to lower a process temperature.
As a means for solving the problem, a technique for obtaining a crystalline silicon film at a lower process temperature has been researched.
This process is called a low temperature process correspondingly to a process (high temperature process) for manufacturing the high temperature polysilicon.
A crystalline silicon film manufactured through this low temperature process is called a low temperature polysilicon.
Techniques for manufacturing low temperature polysilicon films are roughly divided into a method of using laser irradiation and a method of heating.
In the method of using laser irradiation, since laser light is directly absorbed in the vicinity of the surface of an amorphous silicon film, the surface of the amorphous silicon film is merely heated instantaneously and the entire surface is not heated. Thus, the irradiation of laser light can be made a step substantially not accompanied with heating.
However, this method has a problem in the stability of a laser oscillator, and a problem in application to a wide area. Moreover, the crystallinity of an obtained crystalline silicon film is not sufficient.
On the other hand, in the present circumstances, a required crystalline silicon film can not be obtained by a heat treatment at a temperature against which a glass substrate can withstand.
As a technique for improving these current problems, there is a technique disclosed in Japanese Patent Unexamined Publication No. Hei. 6-268212 by the same assignee as the present application.
According to this technique, a metallic element, typified by nickel, for promoting crystallization of silicon is held on the surface of an amorphous silicon film, and then a heat treatment is carried out, so that a crystalline silicon film having required crystallinity may be obtained at a temperatures lower than before at a temperature against which a glass substrate can withstand.
This crystallizing technique using nickel is useful since a crystalline silicon film having required crystallinity can be obtained by a heat treatment at such a low temperature that a glass substrate can withstand.
However, it is impossible to prevent nickel used for crystallization from remaining in an active layer, which causes instability of characteristics and lowering of reliability of a TFT.